We have continued our studies of the insulin-like growth factor/somatomedin, rat IGF-II, a polypeptide synthesized by a cloned line of rat liver cells (BRL-3A). During the past year, progress has been made in the following areas: (1) demonstration of pre-pro-rIGF-II and pro-rIGF-II biosynthetic precursors in cell-free translation and biosynthetic labeling experiments; (2) elucidation of the structure of pro-rIGF-II by molecular cloning and by radiosequencing of pro-rIGF-II; (3) demonstration of two IGF receptor subtypes that differ in binding specificity and in subunit structure; (4) demonstration that insulin and IGF receptors, mediate the same biological responses in hepatocytes, chondrocytes, myocytes, fibroblasts, and embryonal carcinoma cells; (5) demonstration that the beta-subunit of the type I IGF receptor of BRL-3A2 cells has intrinsic tyrosine kinase activity which is activated by IGF-dependent autophosphorylation; (6) demonstration that type I but not type II IGF receptors are selectively downregulated in a mouse muscle cell line; (7) demonstration that insulin acutely increases the number of type II IGF receptors on the surface of intact rat adipocytes by redistribution of cycling receptors from a large intracellular pool; (8) demonstration that insulin desensitizes rat hepatoma cells to IGF-stimulated IGF-receptor-mediated action at a step distal to hormone binding; (9) demonstration that IGF-II levels are high in fetal and early neonatal rats, and fall in older animals; (10) demonstration that rIGF-II mRNA levels in rat liver exhibit similar developmental regulation; (11) demonstration that placental lactogen induces IGF-II synthesis by fetal/neonatal rat fibroblasts, whereas IGF-I synthesis by adult fibroblasts is induced by growth hormone and placental lactogen; (12) characterization of growth hormone-dependent IGF carrier proteins in human serum and fetal and postnatal rat serum; and (13) demonstration that chemically synthesized hybrid molecules containing the A chain of insulin and the B-domain of IGF-I bind to IGF carrier proteins but not to type II IGF receptors.